I agree, the problem was more Lucas than any of the actors.
Portman wasn't the only one to have a poor performance. Liam Neeson, Ewan McGregor, Samuel Jackson, etc. all were underwhelming.
If you have the time, it's worth watching the video reviews of the movie by
Mr Plinkett
They're long, but are funny as hell.
He also points out that there are huge parts of the prequels that consist of "walking and talking" or 'sitting and talking", with the camera shots simply switching from one head shot to another.
He also contrasts this with IV through VI.
One example is the scene where one of the Empire officers walks into Vader's room where we see for just a moment his face with the helmet off.
He points out how with one 10 second bit of film he tells more than 30 minutes of "walking and talking".
How the audience see's his face, knows that he's been injured, and that's why he wears the helmet.
Far more interesting a way than the "walking and talking" and taking advantage of just what film provides: a visual pallet to tell a tale.
None of that exists in the prequels.
